Use multiple linear segments to better interpolate the dBm
for the signal strength.
The table that converts from linear strength to dB was
empirically determinated with the help of a signal generator
(DTA-2111).
The entries from -35dBm to -22.5dBm were taken using just
the signal generator and the board.
For the entries from -36dBm to -51dBm, a 16 dB tap was used,
in order to extend its range.
Signals below to -51dBm are just linearly interpolated.
Signed-off-by: Mauro Carvalho Chehab <m.chehab@xxxxxxxxxxx>
---
drivers/media/dvb-frontends/dib8000.c | 111 ++++++++++++++++++++++++++++------
1 file changed, 92 insertions(+), 19 deletions(-)
diff --git a/drivers/media/dvb-frontends/dib8000.c b/drivers/media/dvb-frontends/dib8000.c
index faf469d1d437..7b10b73befbe 100644
--- a/drivers/media/dvb-frontends/dib8000.c
+++ b/drivers/media/dvb-frontends/dib8000.c
@@ -3843,35 +3843,108 @@ static const struct per_layer_regs per_layer_regs[] = {
{ 556, 581, 583 },
};
+struct linear_segments {
+ unsigned x;
+ signed y;
+};
+
+/*
+ * Table to estimate signal strength in dBm.
+ * This table was empirically determinated by measuring the signal
+ * strength generated by a DTA-2111 RF generator directly connected into
+ * a dib8076 device (a PixelView PV-D231U stick), using a good quality
+ * 3 meters RC6 cable and good RC6 connectors.
+ * The real value can actually be different on other devices, depending
+ * on several factors, like if LNA is enabled or not, if diversity is
+ * enabled, type of connectors, etc.
+ * Yet, it is better to use this measure in dB than a random non-linear
+ * percentage value, especially for antenna adjustments.
+ * On my tests, the precision of the measure using this table is about
+ * 0.5 dB, with sounds reasonable enough.
+ */
+static struct linear_segments strength_to_db_table[] = {
+ { 55953, 108500 }, /* -22.5 dBm */
+ { 55394, 108000 },
+ { 53834, 107000 },
+ { 52863, 106000 },
+ { 52239, 105000 },
+ { 52012, 104000 },
+ { 51803, 103000 },
+ { 51566, 102000 },
+ { 51356, 101000 },
+ { 51112, 100000 },
+ { 50869, 99000 },
+ { 50600, 98000 },
+ { 50363, 97000 },
+ { 50117, 96000 }, /* -35 dBm */
+ { 49889, 95000 },
+ { 49680, 94000 },
+ { 49493, 93000 },
+ { 49302, 92000 },
+ { 48929, 91000 },
+ { 48416, 90000 },
+ { 48035, 89000 },
+ { 47593, 88000 },
+ { 47282, 87000 },
+ { 46953, 86000 },
+ { 46698, 85000 },
+ { 45617, 84000 },
+ { 44773, 83000 },
+ { 43845, 82000 },
+ { 43020, 81000 },
+ { 42010, 80000 }, /* -51 dBm */
+ { 0, 0 },
+};
+
+static u32 interpolate_value(u32 value, struct linear_segments *segments,
+ unsigned len)
+{
+ u64 tmp64;
+ u32 dx;
+ s32 dy;
+ int i, ret;
+
+ if (value >= segments[0].x)
+ return segments[0].y;
+ if (value < segments[len-1].x)
+ return segments[len-1].y;
+
+ for (i = 1; i < len - 1; i++) {
+ /* If value is identical, no need to interpolate */
+ if (value == segments[i].x)
+ return segments[i].y;
+ if (value > segments[i].x)
+ break;
+ }
+
+ /* Linear interpolation between the two (x,y) points */
+ dy = segments[i - 1].y - segments[i].y;
+ dx = segments[i - 1].x - segments[i].x;
+
+ tmp64 = value - segments[i].x;
+ tmp64 *= dy;
+ do_div(tmp64, dx);
+ ret = segments[i].y + tmp64;
+
+ return ret;
+}
+
static int dib8000_get_stats(struct dvb_frontend *fe, fe_status_t stat)
{
struct dib8000_state *state = fe->demodulator_priv;
struct dtv_frontend_properties *c = &state->fe[0]->dtv_property_cache;
int i, lock;
- u64 tmp;
u32 snr, val;
+ s32 db;
u16 strength;
/* Get Signal strength */
dib8000_read_signal_strength(fe, &strength);
-
- /*
- * Estimate it in dBm
- * This calculus was empirically determinated by measuring the signal
- * strength generated by a DTA-2111 RF generator directly connected into
- * a dib8076 device. The real value can actually be different on other
- * devices, depending if LNA is enabled or not, if diversity is enabled,
- * etc.
- */
- if (strength == 65535) {
- c->strength.stat[0].svalue = -22000;
- } else {
- tmp = strength * 25000L;
- do_div(tmp, 11646);
- c->strength.stat[0].svalue = tmp - 142569;
- if (c->strength.stat[0].svalue > -22000)
- c->strength.stat[0].svalue = -22000;
- }
+ val = strength;
+ db = interpolate_value(val,
+ strength_to_db_table,
+ ARRAY_SIZE(strength_to_db_table)) - 131000;
+ c->strength.stat[0].svalue = db;
/* Check if 1 second was elapsed */
if (!time_after(jiffies, state->get_stats_time))
--
1.8.3.1
--
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